1. Field of the Invention
The invention relates to a liquid crystal composition suitable for use in an active matrix (AM) device, and an AM device containing the composition. More specifically, the invention relates to a liquid crystal composition having a negative dielectric anisotropy, and also relates to a device of an in-plane switching (IPS) mode or a vertical alignment (VA) mode.
2. Related Art
In a liquid crystal display device, classification based on an operating mode of liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA), and so forth. Classification based on a driving mode includes a passive matrix (PM) and an active matrix (AM). PM is further classified into static, multiplex and so forth, and AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth. TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to the production process. Classification based on the light source includes a reflection type utilizing the natural light, a transmission type utilizing a backlight, and a semi-transmission type utilizing both the natural light and a backlight.
These devices contain a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to obtain an AM device having good general characteristics. Table 1 below summarizes the relationship between the general characteristics. The general characteristics of the composition will be explained further based on a commercially available AM device. A temperature range of a nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is approximately 70° C. or more and a desirable minimum temperature is approximately −10° C. or less. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying a moving image. Accordingly, a low viscosity of the composition is desirable. A low viscosity at a low temperature is more desirable.
TABLE 1General Characteristics of a Liquid Crystal Composition and AM DeviceGeneral Characteristics of aGeneralNoCompositionCharacteristics of an AM Device1Temperature range of theUsable temperature range is widenematic phase is wide2Viscosity is low1) and rotationResponse time is shortviscosity is low3Optical anisotropy is suitableContrast ratio is large4Dielectric anisotropy isThreshold voltage is low, electricpositively or negatively largepower consumption is small andcontrast ratio is large5Specific resistance is largeVoltage holding ratio is large andcontrast ratio is large6It is stable with ultravioletService life is longlight and heat1)A liquid crystal composition can be injected into a cell in a short time.
The optical anisotropy of the composition relates to the contrast ratio of the device. A product (Δn·d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed to maximize the contrast ratio. A suitable value of the product depends on the kind of operation mode. In a device having a VA mode, a suitable value is in a range of from approximately 0.30 μm to approximately 0.40 μm, and in a device having an IPS mode; a suitable value is in a range of from approximately 0.20 μm to approximately 0.30 μm. In this case, a composition having a large optical anisotropy is desirable for a device having a small cell gap. A large dielectric anisotropy of the composition contributes to a low threshold voltage, small electric power consumption and large contrast ratio. Accordingly, a large dielectric anisotropy is desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, a composition having a large specific resistance is desirable at room temperature and also at a high temperature in the initial stage. A composition having a large specific resistance is desirable at room temperature and also at a high temperature after it has been used for a long time. The stability of the composition to ultraviolet light and heat relates to the service life of the liquid crystal display device. In the case where the stability is high, the device has a long service life. These characteristics are desirable for an AM device used in a liquid crystal projector, a liquid crystal television and so forth.
In an AM device having a TN mode, a composition having a positive dielectric anisotropy is used. In an AM device having a VA mode, a composition having a negative dielectric anisotropy is used. In an AM device having an IPS mode, a composition having a positive or negative dielectric anisotropy is used. A liquid crystal composition having a negative dielectric anisotropy is disclosed in the following document.
A conventional composition is disclosed in JP H2-4723 A/1990.
A desirable AM device is characterized as having a usable temperature range that is wide, a response time that is short, a contrast ratio that is large, a threshold voltage that is low, a voltage holding ratio that is large, a service life that is long, and so forth. Even one millisecond shorter response time is desirable. Thus, a composition having characteristics such as a high maximum temperature of a nematic phase, low minimum temperature of a nematic phase, low viscosity, large optical anisotropy, large positive or negative dielectric anisotropy, large specific resistance, high stability to ultraviolet light, high stability to heat, and so forth is especially desirable.